1. Field of the Invention
The present invention relates to a fluorescent lamp, and particularly to a bulb-shaped fluorescent lamp.
2. Related Background Art
In the last few years, the consciousness for saving energy has increased, and as an alternative low power light source that takes the place of low efficiency bulbs, fluorescent lamps, in particular, bulb-shaped fluorescent lamps, have been developed and used actively.
The bulb-shaped fluorescent lamp basically has an integrated structure, as shown in FIG. 4. The bulb-shaped fluorescent lamp includes a combination of a fluorescent arc tube 18 in which glass tubes each having coiled electrodes 19 and 20 on end portions are bent or jointed to form a series of discharge paths, and an electronic ballast 21 that is a circuit for lighting the fluorescent arc tube 18 stably. The bulb-shaped fluorescent lamp further is provided with a bulb base 22. Typically, the fluorescent arc tube 18 is disposed in an outer tube glass bulb 23, and the electronic ballast 21 is disposed in a resin case 24.
A conventional bulb-shaped fluorescent lamp has addressed important issues such as improvement in characteristics (e.g., lamp efficiency and its life). In addition, it has been attempted to reduce the conventional bulb-shaped fluorescent lamp in its size and weight so as to use it as an alternative to a bulb. As a result, as shown in FIG. 4, the electronic ballast 21 for high frequency lighting is employed currently as a main ballast in the bulb-shaped fluorescent lamp, because it is superior in both aspects such as the improvement in lamp characteristics and reduction in its size and weight.
As shown in FIG. 5, the general purpose lighting circuit of the above-described conventional electronic ballast 21 includes an inverter circuit portion 25, an inductor 26 connected in series to the fluorescent arc tube 18 and a capacitor 27 connected in parallel to the fluorescent arc tube 18. In addition, a positive temperature characteristic resistive element (PTC) 28 connected in parallel to the fluorescent arc tube 18 is installed as in almost all circuits.
Hereinafter, an operation of lighting a lamp by the conventional lighting circuit will be described.
(a) First, when a power supply 29 is turned on, a preheat current sufficient for starting the arc tube flows through the coiled electrodes 19 and 20 via the capacitor 27 and the positive temperature characteristic resistive element 28.
(b) Next, when the resistance of the positive temperature characteristic resistive element 28 becomes high due to a rise in temperature, a so-called resonant voltage (peak value: 700 V to 1000V) of the inductor 26 and the capacitor 27 is applied to the fluorescent arc tube 18, whereby the fluorescent arc tube 18 starts.
(c) Thereafter a predetermined lamp current flows through the fluorescent arc tube 18, and the lamp starts to light steadily. During steady lighting of the lamp, a current also continuously flows through the capacitor 27 via the coiled electrodes 19 and 20.
As described above, the conventional lighting circuit in which the capacitor 27 is connected in parallel to the fluorescent arc tube 18 has a relatively simple configuration. In addition, it is characterized in that the coiled electrodes 19 and 20 are preheated sufficiently and supplied with a predetermined starting voltage when the lamp starts. In this respect, it is considered to be an inexpensive and reliable circuit.
Moreover, as shown in FIG. 4, during assembly of the circuit parts of the electronic ballast 21, main parts including the capacitor 27 are disposed and mounted on the surface of a printed circuit board 30 on the base 22 side. This allows the printed circuit board 30 to thermally insulate the main parts from a heat source or the fluorescent arc tube 18 during lighting of the lamp, thereby suppressing a temperature rise in the main parts.
As a recent trend of a bulb-shaped fluorescent lamp, a high watt-type lamp of 20 W or more (e.g., 22 W to 25 W) has been developed as an alternative to a 100 W bulb, along with a low watt-type lamp of 13 W as an alternative to a 60 W bulb. Since a 60 W bulb is the same as a 100 W bulb in shape, in the development of this high watt-type lamp, it also has been attempted to miniaturize the lamp as an alternative to a bulb. That is to say, a high watt-type lamp is required while it is kept as small as possible.
The present inventors employed the electronic ballast 21 composed of a basic characteristic circuit of the prior art, as shown in FIG. 4 and worked toward the development of a similar small high watt-type lamp. As a result, it was found that a temperature rise particularly in the circuit parts of the electronic ballast cannot be avoided, which results in an occurrence of circuit failure and a short life of the lamp. Further, the analysis of the lamp with a short life revealed that the circuit failure is caused mainly by the damage of the capacitor 27 connected in parallel to the fluorescent arc tube 18 in FIG. 4 due to its temperature rise.
It is an object of the present invention to solve the above-described problems by providing a bulb-shaped fluorescent lamp adopting an inexpensive and reliable electronic ballast.
To achieve the above object, the bulb-shaped fluorescent lamp of the present invention includes a fluorescent arc tube, an electronic ballast for lighting and driving the fluorescent arc lamp, and a base for providing power to the electronic ballast, which are arranged in this order. The electronic ballast includes an inductor connected in series to the fluorescent arc tube and at least two capacitors constituting a resonant circuit with the inductor and connected in parallel to the fluorescent arc tube.
In this bulb-shaped fluorescent lamp, it is preferable that at least two capacitors are disposed on the surface of a printed board constituting the electronic ballast on the base side, and their bodies are disposed stepwise without being opposed to each other. In this case, it also is preferable that as the capacitance of at least two capacitors becomes larger, their bodies are spaced further from the fluorescent arc tube and disposed stepwise.
According to the above-described constitution, the bulb-shaped fluorescent lamp adopting an inexpensive and reliable electronic ballast can be realized, in which the surface temperature of at least two capacitors during steady lighting of the lamp is kept at not more than a guaranteed upper limit operating temperature, resulting in reduction of damage to the at least two capacitors during the use of the lamp.